In the manufacture of semiconductor devices, ion implantation is used to dope semiconductors with impurities or dopants. Ion beam implanters are used to treat silicon wafers with an ion beam, in order to produce n or p type extrinsic material doping or to form passivation layers during fabrication of an integrated circuit. When used for doping semiconductors, the ion beam implanter injects a selected ion species to produce the desired extrinsic material. Implanting ions generated from source materials such as antimony, arsenic or phosphorus results in “n type” extrinsic material wafers, whereas if “p type” extrinsic material wafers are desired, ions generated with source materials such as boron, gallium or indium may be implanted.
Typical ion beam implanters include an ion source for generating positively charged ions from ionizable source materials. The generated ions are formed into a beam and directed along a predetermined beam path to an implantation station. The ion beam implanter may include beam forming and shaping structures extending between the ion source and the implantation station. The beam forming and shaping structures maintain the ion beam and bound an elongated interior cavity or passageway through which the beam passes en route to the implantation station. When operating an implanter, this passageway can be evacuated to reduce the probability of ions being deflected from the predetermined beam path as a result of collisions with gas molecules.
The mass of an ion relative to the charge thereon (e.g., charge-to-mass ratio) affects the degree to which it is accelerated both axially and transversely by an electrostatic or magnetic field. Therefore, the beam which reaches a desired area of a semiconductor wafer or other target can be made very pure since ions of undesirable molecular weight will be deflected to positions away from the beam and implantation of other than desired materials can be avoided. The process of selectively separating ions of desired and undesired charge-to-mass ratios is known as mass analysis. Mass analyzers typically employ a mass analysis magnet creating a dipole magnetic field to deflect various ions in an ion beam via magnetic deflection in an arcuate passageway which will effectively separate ions of different charge-to-mass ratios.
Dosimetry is the measurement of the number of ions per unit area implanted in a wafer or other workpiece. In controlling the dosage of implanted ions, closed loop feedback control systems typically are utilized in order to dynamically adjust the implantation to achieve uniformity in the implanted workpiece. In one example, a control system utilizes real-time current monitoring to control the slow scan velocity. A Faraday disk or Faraday cup periodically measures the beam current and adjusts the slow scan speed to ensure constant dosing. Frequent measurement allows the dose control system to respond quickly to changes in beam current. The Faraday cup may be stationary, well shielded, and located close to the wafers, making it sensitive to the beam current actually dosing the wafers.
Faraday cups measure only the electric current. Interactions between the ion beam and gases evolved, such as from photo resist, during implant can result in a change in charge state of some of the beam ions, most commonly from a singly-charged positive ion to a neutral atom. As a result, the measured flux or beam current can misrepresent the actual beam current or flux. Implanted neutrals contribute to the dose received by a wafer, but are not measurable by a faraday cup. As a result, the wafer can be overdosed and/or have substantial dose non-uniformities.
A conventional mechanism to account for such variations employs obtaining pressure readings in addition to the faraday cup readings. Both the pressure readings and the faraday cup measurements are then employed to adjust beam current. However, the amount of correction applied to the faraday current reading depends, for example, on the beam energy and ion species. In addition the magnitude of the correction can be large, which necessitates accurate determination of compensation factors and precise pressure measurements. In practice it can be difficult to use such methods to obtain the desired dose uniformity and repeatability.